1. Field of the Invention
The present invention relates to sensor coils for fiber optic gyroscopes. More particularly, this invention pertains to a sensor coil for use in a fiber optic gyroscope that is mounted upon a spool designed for anisotropic thermal expansion characteristics to thereby mimic the coil and thereby reduce bias error-inducing mechanical stresses.
2. Description of the Prior Art
A fiber optic gyroscope 10 comprises the following main components as illustrated in the schematic diagram of FIG. 1: a light source 12; one or two beamsplitters 14, 15 (fiber optic directional couplers and/or an integrated-optics Y-junctions); a fiber optic coil 16; a polarizer 18 (and sometimes one or more depolarizers); and a detector 20. Light from the light source 12, after passing through polarizer 18, is split by the beamsplitter 15 into copropagating and counterpropagating waves that travel through the sensing coil 16. Associated electronics 22 measures the phase relationships between the two interfering, counterpropagating beams of light that emerge from the opposite ends of the coil 16 and impinge upon the detector 20 after passing through the beamsplitter 14. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. These are both time-varying and unevenly distributed throughout the coil and induce variations in index of refraction and length that each counterpropagating wave encounters as it travels through the coil. The phase shifts imposed upon the two waves due to environmental factors can be unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
One approach to reducing the sensitivity arising from environmental factors has involved the use of symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the winding is located at the innermost layer while the two ends of the coil are located at the outermost layers.
N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities in "Compensation of Linear Sources of Non-Reciprocity in Sagnac Interferometers", Fiber Optics and Laser Sensors I, Procs. SPIE, v. 412, p.261 (1983). Furthermore, U.S. Pat. No. 4,793,708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by duopole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding.
U.S. Pat. No. 4,856,900 of Ivancevic entitled "Quadruple-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereof" teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent to end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns that climb between connecting layers. Both of the aforementioned United States patents are the property of the assignee herein.
Pending U.S. Pat. No. 5,333,214 of Huang et al. entitled "Apparatus For Reducing Magnetic Field-Induced Bias Errors in a Fiber Optic Gyroscope" addresses the suppression of bias errors induced by the Faraday effect in a sensor coil exposed to a magnetic field. The invention disclosed in that patent (property of the assignee herein) teaches the use and design of compensator loops for counteracting the effects of both radially and axially-directed magnetic fields. In either case, a predetermined degree of twist of a preselected fiber twist mode is imposed upon the compensator loop to create a counteracting-corrective Faraday effect.
U.S. Pat. No. 5,371,593 of Cordova et al. entitled "Sensor Coil For Low Bias Fiber Optic Gyroscope", also property of the assignee herein, addresses additional problems related to environmental factors. While acknowledging that the design of the sensor coil can impact the gyro's random walk, bias stability, bias temperature sensitivity, bias temperature-ramp sensitivity, bias vibration sensitivity, bias magnetic sensitivity, scale factor temperature sensitivity, scale factor linearity and input axis temperature sensitivity, the device disclosed in that application discloses a coil for which windings are potted in an adhesive material of a predetermined composition. Careful selection of the potting material (particularly in terms of modulus of elasticity) results in reduction of vibration-induced bias, coil cracking, degradation of h-parameter and temperature-ramp bias sensitivity. The coil is formed on a spool of carbon composite material whose coefficient of thermal expansion approximates that of the overlying fiber windings. In addition, this application discloses that the close matching of the thermal expansion characteristics of the spool and the fiber windings as well as proper selection of the coil potting material will minimize the Shupe-like bias caused by thermal stress that would be otherwise exerted by a standard metallic spool.
Conventional support and spool designs, which feature a substantially-cylindrical hub sandwiched between a pair of end flanges, are difficult to "match" to the potted coil. This is due to the asymmetry of expansions of such coils in response to temperature change. The coefficient of thermal expansion of a potted coil in the axial direction is often on the order of ten (10) to one-hundred (100) times that of the radial direction. Unfortunately, spools of conventional design and material composition exhibit isotropic thermal expansion characteristics. This relative imbalance introduces bias errors through coil stressing and creates bonding and cracking problems. For example, in a spool-and-coil arrangement in which the material of the hub of the spool closely approximates the radial coefficient of thermal expansion of the potted coil, the axial expansion of the coil will exceed that of the hub. As a result, significant axial compression of the coil can occur when the temperature rises since axial expansion of the potted coil is limited by a relatively "fixed" separation distance between the spool's end flanges. Further, the stressing due to differential thermal expansion coefficients at the coil-hub interface can result in either rupture or in coil cracking. On the other hand, in a spool fabricated of material closely matching the axial coefficient of thermal expansion of the potted coil, one may expect the relatively-larger radial expansion of the hub in response to temperature change to degrade performance by squeezing the fiber of the coil whose radial dimension is relatively fixed.
Pending U.S. patent application Ser. No. 08/116,376 of Patterson entitled "Flange-Supported Sensor Coil For a Fiber Optic Gyroscope", property of the assignee herein, discloses a spool designed to address the thermally-induced Shupe bias that results from the above-mentioned thermal incompatibility of conventional spool designs with the asymmetric radial and axial thermal expansion coefficients of potted sensor coils. That application discloses a spool that consists of a single mounting flange and an interior hub. The coil is mounted upon with axis transverse to the plane of the flange. In an alternative embodiment, the coil is split into sections that lie atop and at the bottom of the flange. The coil is free to expand axially because the interior of the coil is separated from the hub by a finite distance. The much smaller radial coefficient of thermal expansion of the coil assures that the separation from the hub needn't be excessive and that undue thermal stressing is not experienced at the coil-flange interface. While the above device exhibits good thermal performance characteristics, it is subject to vibration-induced bias effects that result from the quasi-free standing arrangement of the coil relative to the spool. Such bias effects can become particularly acute in an environment that includes vibrations at the resonance frequency of the potted coil.
Pending U.S. patent application Ser. No. 08/299,585 of Bilinski, et al., property of the Assignee herein, addresses the problem of environmental vibration by providing a hub for a spool of the single-flange type whose outer surface is characterized by a low coefficient of friction. This enables one to wind the coil directly onto the hub and thereby avoid the perils associated with a quasi-free standing arrangement. The integral structure of the coil-plus-spool hub is characterized by much higher resonance frequencies, out of the range of environmental vibrations, than a quasi-free standing coil. Unfortunately, the operative mechanism of such device for overcoming the effects of axial coil expansion relies upon the ability of the potted coil to slide freely upon the surface of the hub. Even slight imperfections in the surface of the hub can occasion a deleterious so-called "stick and slip" phenomenon. This effect can cause irregular and quasi-periodic stressing of the coil fiber resulting in unpredictable bias errors in the gyro output.